It's pretty good, man. Using a circular saw to do your cuts is actually pretty hard. That's why I LOOOVE my Tablesaw. However, that's a nice build and a nice use of space. I personally would've tried to get as close to the edge as possible. I've also seen people who make an air-tight top baffle for those type of trunks so it's just one piece of wood with the sub in it, and any kind of weather stripping at the edges to make it air tight. Pretty basic, but effective (if you do it correctly).
EDIT:: Am I the only one that uses Titebond I? I've seen people use Titebond III, and you use Titebond II, what's the difference? I just grabbed one and left.